The present invention relates to a drilling assembly for drilling a borehole into an earth formation, comprising a drill string extending into the borehole and a jetting device arranged at the lower end of the drill string. The jetting device ejects a high velocity stream of drilling fluid against the rock formation so as to erode the rock and thereby to drill the borehole. In order to improve the rate of penetration of the drill string it has been proposed to mix abrasive particles into the jet stream.
One such system is disclosed in U.S. Pat. No. 3,838,742 wherein a drill string is provided with a drill bit having a number of outlet nozzles. Drilling fluid containing abrasive particles is pumped via the drill string through the nozzles to produce high velocity jets impacting against the borehole bottom. The abrasive particles accelerate the erosion process compared to jetting of drilling fluid only. The rock cuttings are entrained into the stream which returns through the annular space between the drill string and the borehole wall to surface. After removal of the rock cuttings from the stream, the pumping cycle is repeated. A drawback of the known system is that continuous circulation of the abrasive particles through the pumping equipment and the drill string leads to accelerated wear of these components. Another drawback of the known system is that constraints are imposed on the rheological properties of the drilling fluid, for example a relatively high viscosity is required for the fluid to transport the abrasive particles upwardly through the annular space.
It is an object of the invention to provide an improved drilling assembly for drilling a borehole into an earth formation, which overcomes the drawbacks of the known system and which provides an increased rate of penetration without accelerated wear of the drilling assembly components.
In accordance with the invention there is provided a drilling assembly for drilling a borehole into an earth formation, comprising a drill string extending into the borehole and a jetting device arranged at a lower part of the drill string, the jetting device being provided with a mixing chamber having a first inlet in fluid communication with a drilling fluid supply conduit, a second inlet for abrasive particles and an outlet which is in fluid communication with a jetting nozzle arranged to jet a stream of abrasive particles and drilling fluid against at least one of the borehole bottom and the borehole wall, the jetting device further being provided with an abrasive particles recirculation system for separating the abrasive particles from the drilling fluid at a selected location where the stream flows from said at least one of the borehole bottom and the borehole wall towards the upper end of the borehole and for supplying the separated abrasive particles to the second inlet.
The abrasive particle recirculation system separates the abrasive particles from the stream after impact of the stream against the rock formation, and returns the abrasive particles to the mixing chamber. The remainder of the stream which is, apart from the drill cuttings, substantially free of abrasive particles, returns to surface and is recycled through the drilling assembly after removal of the drill cuttings. It is thereby achieved that the abrasive particles circulate through the lower part of the drilling assembly only while the drilling fluid which is substantially free of abrasive particles circulates through the pumping equipment, and that no constraints are imposed on the rheological properties of the drilling fluid regarding transportation of the abrasive particles to surface.
Suitably the recirculation system includes means for creating a magnetic field in the stream, and the abrasive particles include a material subjected to magnetic forces induced by the magnetic field, the magnetic field being generated such that the abrasive particles are separated from the drilling fluid by said magnetic forces. The means for creating the magnetic field comprises, for example, at least one magnet.
In a preferred embodiment, the drill string is at the lower end thereof provided with a drill bit, and the jetting nozzle is arranged to jet the stream of abrasive particles and drilling fluid against the wall of the borehole as drilled by the drill bit so as to enlarge the borehole diameter to a diameter significantly larger than the diameter of the drill bit. By drilling the borehole using the drill bit and enlarging the borehole diameter to a diameter significantly larger than the diameter of the drill bit, a tubular such as a casing or a liner can be installed in the borehole while the drill string is still present in the borehole. The drill string and drill bit can thereafter be retrieved to surface through the tubular.
The tubular to be installed in the borehole can be formed by the drill string, in which case the drill string has an inner diameter larger than the outer diameter of the drill bit, the drill bit being detachable from the drill string and being provided with means for detaching the drill bit from the drill string and for retrieving the drill bit through the drill string to surface.